Flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. Generally, engine braking systems may control the flow of exhaust gas to incorporate the principles of compression-release type braking, exhaust gas recirculation, exhaust pressure regulation, and/or bleeder type braking.
The operation of a compression-release type engine brake, or retarder, is well known. During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
The principles of exhaust gas recirculation (EGR) are also well known. An EGR system allows a portion of the exhaust gases to flow back into the engine cylinder and is primarily used to reduce the amount of NOx created by the engine during positive power operations. An EGR system can also be used to control the pressure and temperature in the exhaust manifold and engine cylinder during engine braking cycles. Generally, there are two types of EGR systems, internal and external. External EGR systems recirculate exhaust gases back into the engine cylinder through an intake valve(s). Internal EGR systems recirculate exhaust gases back into the engine cylinder through an exhaust valve(s).
Furthermore, control of EGR may be achieved by selectively varying the levels of exhaust back pressure using Exhaust Pressure Regulation (EPR). By controlling EGR with EPR, the levels of pressure and temperature in the exhaust manifold and engine cylinders may be maintained such that optimal degrees of engine braking are attained at any engine speed. An example of a method and system for optimizing engine braking using EGR and EPR is provided by the disclosure of Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001) for Method and System For Controlled Exhaust Gas Recirculation in an Internal Combustion Engine With Application to Retarding and Powering Function, which is hereby incorporated by reference.
The operation of a bleeder type engine brake has also long been known. During engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke.
Usually, the initial opening of the braking valve(s) in a bleeder braking operation is far in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake requires much lower force to actuate the valve(s) due to early valve actuation, and generates less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Moreover, bleeder brakes often require fewer components and can be manufactured at lower cost. Thus, an engine bleeder brake can have significant advantages.
Despite these advantages, however, bleeder type engine brakes have not been widely used because they typically produce less braking power than the compression-release type brakes in heavy duty diesel engines with a conventional fixed geometry turbocharger (FGT). This reduced braking power occurs especially at low and moderate engine speeds.
With the introduction of variable geometry turbochargers (VGT), however, bleeder brakes become a more attractive option. Through the use of VGT, both the intake and exhaust manifold pressures may be much higher than those produced using conventional FGT. These increased pressures may correspond to greatly improved bleeder brake performance, especially at low and moderate engine speeds.
The prior art methods and systems do not disclose incorporating VGT to improve bleeder braking performance. For example, Faletti et al., U.S. Pat. No. 6,148,793 (Nov. 21, 2000), discloses a compression-release type braking system utilizing a variable geometry turbocharger, but does not disclose a bleeder braking system using VGT for optimizing engine braking. Similarly, Church et al., U.S. Pat. No. 6,134,890 (Oct. 24, 2000), discloses a method for controlling VGT for providing precise control of turbo boost pressure. The >890 patent does not, however, disclose a system and method utilizing VGT to control exhaust back pressure for improving bleeder braking performance.
Also, Price et al., U.S. Pat. No. 4,395,884 (Aug. 2, 1983) and U.S. Pat. No. 4,474,006 (Oct. 2, 1984), disclose principles similar to those of a variable geometry turbo to control engine braking, but do not disclose methods and systems of engine bleeder braking using VGT. In addition, A. Flotho et al., New Engine Brake Systems for Commercial Vehicles (1999), which is hereby incorporated by reference, discloses a two-stage turbocharger adapted to enhance engine braking, but the geometry of the turbocharger is not variable. Accordingly, there is a significant need for a method and system for engine braking in an internal combustion engine with VGT that captures the inherent advantages of bleeder braking operation and provides improved bleeder braking performance.
Current variable geometry turbochargers typically include a mechanical stop that prevents the geometry (vanes) of the VGT from fully closing. This is so because, once the vanes are fully closed, the significant pressure gradient across the VGT creates control instability and prevents the vanes from being re-opened. The mechanical stop of most VGTs is based on optimum engine positive power operation, and is usually not optimum for engine braking. This means that the increased exhaust manifold pressures created below this position, and, thus, opportunities for improved engine braking, may be forfeited.
None of the prior art methods and systems teach or suggest minimizing the pressure gradient across the VGT to avoid control instability and maximize the VGT geometry variation range. Accordingly, there is an additional need for a method and system for controlling engine braking in an internal combustion engine with VGT that captures the advantages of bleeder braking operation but eliminates or reduces the limitations caused by the mechanical stop features of conventional variable geometry turbochargers. FIG. 1 illustrates an example of a comparison between improved bleeder brake performance results in a full-cycle bleeder braking system with VGT according to the present invention obtained by the present Assignee and performance using conventional VGTs with the mechanical stop limitation.
In addition, the braking performance of a bleeder braking system can be further optimized by using EGR and EPR to tune exhaust back pressure. By combining EGR and EPR with fully operational VGT, the levels of pressure and temperature in the exhaust manifold and engine cylinders may be maintained such that optimal degrees of engine braking are attained at any engine speed. None of the prior art systems and methods, of which the present inventors are aware, teach or suggest this combination.
The systems and methods of the present invention respond to the needs left unanswered by the prior art. The present invention provides systems and methods for improving bleeder brake performance using any combination of turbocharger control, EGR, and EPR. The present invention further provides systems and methods for eliminating or reducing the limitations caused by the mechanical stop features of conventional VGT so that the VGT variation range may be extended for wider application in both engine braking and positive power operations. In addition, the present invention provides improved mechanisms and devices to achieve a bleeder braking cycle.